Technical Field
The invention relates to a laminated lithium primary battery in which the jacket made of laminate film accommodates an electrode assembly, the electrode assembly including negative electrode made of lithium.
Related Art
In recent years, various sheet-like thin electronic devices in which power sources are included are in practical use; for example, electronic papers, Integrated Circuit tags (IC tags), Integrated Circuit cards (smart cards), and electronic key cards. As power sources for these devices, there are laminated lithium primary batteries, which are suitable to be made small and thin. FIGS. 1A and 1B show the basic construction of a laminated lithium primary battery 1. FIG. 1A is an external view of the laminated lithium primary battery 1, and FIG. 1B is an exploded perspective view illustrating the internal construction of the battery 1. In FIG. 1B, some of parts are hatched so as to distinguish them over the other parts. The laminated lithium primary battery 1 has a flat external shape, as shown in FIG. 1A. On one side of a flat, rectangular jacket 11 made of laminate film, a positive terminal plate 23 and a negative terminal plate 33 reach outside of the jacket 11 from the inside of the jacket 11.
As shown in FIG. 1B, the jacket 11 is formed by heat-sealing the periphery of two laminate films (11a and 11b), and an electrode assembly 10 is arranged inside the heat-sealed frame (hereinafter referred to as a sealed part 50); the electrode assembly 10 being formed by stacking a sheet-like positive electrode 20, a sheet-like negative electrode made of lithium (hereinafter referred to as negative electrode lithium 30) and a separator 40 interposed between them. The electrode assembly 10 is enclosed inside the jacket 11 together with electrolytic solution in which non-aqueous organic solvent is used (hereinafter referred to as non-aqueous electrolytic solution).
As shown in drawings, a direction in which the positive electrode 20, the separator 40, and the negative electrode 30 are stacked is defined as an up-and-down direction and the side of the positive electrode 20 in the electrode assembly 10 is defined as “up”. More specifically, the positive electrode 20 is made by applying cathode material 22 in slurry form to a surface of a cathode current collector 21 (made of metal foil, etc.) and drying it. In this example, a strip-like projection is formed in an integrated manner with the cathode current collector 21, and the tip end of the projection reaches outside of the jacket 11. A part of the jacket 11 which is exposed outside is the positive terminal plate 23. To the lower surface of the negative electrode lithium 30, a strip-like metal plate 31 is attached, and a part of the metal plate 31 reaches outside of the jacket to form the negative terminal plate 33. Though the laminated lithium primary battery 1 including a single electrode assembly 10 is described above as an example, a plurality of electrode assemblies 10 may be stacked in the up-and-down direction depending on the output voltage required by external devices, battery capacity, or the like.
A battery (such as a lithium primary battery) having high operating voltage and including combustible non-aqueous electrolytic solution needs, regardless of the construction of the jacket, safety measure specific to rise in the temperature inside the battery. In a lithium primary battery, a separator made of resin material (e.g. polyethylene) is used, and the resin material has a shutdown function to interrupt current between the positive electrode and the negative electrode by melting at high temperature. The following Non Patent Literature 1 describes safety and evaluation criteria of a battery including non-aqueous electrolytic solution, shutdown function of its separator, and the like.    [Non Patent Literature 1] NTT Facilities Research Institute Inc., “Internal Short Circuit in Lithium-ion Batteries and Safety Testing”, [online], [search result on Jan. 22, 2015], Internet <URL:https://www.ntt-fsoken.co.jp/research/pdf/2007_ichi.pdf>
As well known, lithium actively reacts with moisture to release heat. And, moisture is more likely to penetrate from outside into a battery whose jacket is made of laminate film, such as a laminated lithium primary battery, than into a battery in which a metal can is used. A separator having shutdown function has been, therefore, essential for laminated lithium primary batteries.
When lithium reacts with moisture, lithium hydroxide is produced. In the lithium primary batteries, this lithium hydroxide will worsen battery life and dischargeable capacity. In addition, in the laminated lithium primary batteries, its negative electrode lithium is in the form of thin foil and the amount of lithium itself is small. And therefore, battery life will more significantly deteriorate even if a little lithium hydroxide is produced.
Concerning increase in capacity, there is greater demand for laminated lithium primary batteries than for other types of batteries. For example, there is a thin expendable electronic device, such as a smart card, which has an expiry date and will be used for several years, and if laminated lithium primary batteries are used in the foregoing electronic device, it is necessary to securely output stable operating voltage for a long period. But, in a case of a smart card, whose external size is specified by standard, it is impossible to enlarge the external size of its laminated lithium primary battery. Accordingly, a space to accommodate an electrode assembly is provided by enlarging the area of the electrode assembly and narrowing the width of a sealed part. But, narrowing the width of sealing worsens the sealing quality of laminate film, and consequently moisture is more likely to penetrate. In order to realize both good sealing quality and large capacity, it can be reasonably considered that laminate film is welded with a narrower sealing width at higher temperature and higher pressure. But, it has been found that welding the laminate film at a higher temperature than in conventional batteries causes deterioration of shutdown function.
An advantage of the invention is to provide a laminated lithium primary battery which makes it possible to prevent battery life deterioration caused by moisture penetration from outside, and in which safety and increase of battery capacity both can be realized.